In a typical cellular radio system, wireless terminals (also known as mobile stations and/or user equipment units (UEs)) communicate via a radio access network (RAN) to one or more core networks. The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” (UMTS) or “eNodeB” (LTE). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipment units (UE) within range of the base stations.
In some versions of the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a controller node (such as a radio network controller (RNC) or a base station controller (BSC)) which supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). UTRAN is essentially a radio access network using wideband code division multiple access for user equipment units (UEs). In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. Specifications for the Evolved Packet System (EPS) have completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) (also known as the Long Term Evolution (LTE) radio access) and the Evolved Packet Core (EPC) (also known as System Architecture Evolution (SAE) core network). E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to radio network controller (RNC) nodes. In general, in E-UTRAN/LTE the functions of a radio network controller (RNC) node are distributed between the radio base stations nodes (eNodeB's in LTE) and the core network. As such, the radio access network (RAN) of an EPS system has an essentially “flat” architecture comprising radio base station nodes without reporting to radio network controller (RNC) nodes.
Long Term Evolution (LTE) uses single-carrier frequency-division multiple access (SC-FDMA) in an uplink direction from the wireless terminal to the eNodeB. SC-FDMA is advantageous in terms of power amplifier (PA) efficiency since, e.g., the SC-FDMA signal has a smaller peak-to-average ratio than an orthogonal frequency division multiple access (OFDM) signal. However, SC-FDMA gives rise to inter-symbol interference (ISI) problem in dispersive channels. Address inter-symbol interference (ISI) can enable SC-FDMA to improve power amplifier efficiency without sacrificing performance.
FIG. 1 shows an example implementation of the aforementioned the Evolved Packet System as comprising for example the E-UTRAN radio access network and the Mobility Management Entity (MME) in the EPC. The mobility management entity (MME) handles various control functions. The nodes and LTE/SAE or EPS architecture of FIG. 1 and other architecture scenarios are understood with reference to 3GPP TS 23.401, which is incorporated herein by reference and which provides, e.g., a system architecture description.
In some of its implementations, the E-UTRAN may comprise a number of different base stations, e.g., eNodeBs (eNBs) as shown in FIG. 2. S1-MME interface/reference point is used for control signaling between the eNBs and the mobility management entity (MME). An eNB may have S1 links to multiple MMEs in case the MME pool concept is used. The user plane data goes via the Serving GateWay (SGW) on S1-U interface/reference point. Between eNBs the X2 interface/reference point is used.
In a cellular network there will always be areas with high traffic, i.e. high concentration of users. In those areas it may be desirable to deploy additional capacity to keep the user satisfaction. In this respect, a number of approaches are possible: (i) increase the density of their existing macro base stations; (ii) increase the cooperation between macro base stations; or (iii) deploy smaller base stations in areas where high data rates are needed within a macro base stations grid. This last option involves deploying nodes with lower output power and thus which cover a smaller area in order to concentrate the capacity boost on a smaller area.
There will also be areas with bad coverage where there is a need for coverage extension, and again one way to do that is to deploy a node with low output power to concentrate the coverage boost in a small area
Such a smaller radio base station is also called a “femto radio base station” and/or a “home radio base station” and/or “pico radio base station” and/or “micro radio base station” in some contexts. All such small radio base stations are collectively referred to herein as a low power base station in view of the fact that such base stations, in their communications with a wireless terminal, have less output power than a macro base station. For example, whereas a macro base station may transmit with a power of approximately 20 watts, a low power base station may transmit with a power of approximately one watt.
One argument for choosing nodes with lower output power in the above cases is that the impact on the macro network can be minimized, e.g., there is a smaller area where the macro network may experience interference.
Currently there is a strong drive in the industry in the direction towards the use of such low power nodes. The different terms used for a type of network that deploys both macro base station node and low power nodes include “Heterogeneous Networks”, “Multilayer Networks”, and (in abbreviated fashion) “HetNets”. The layer comprising smaller, low power base stations is sometimes termed a “micro” or “pico” or “femto” layer.
In the above regard, FIG. 3 shows portions of an example heterogeneous radio access network. FIG. 3 illustrates a macro base station (the high tower) 24 which provides a wide area coverage (also called macro cell), as well as examples of low power nodes that are deployed to provide small area capacity/coverage. In other words, FIG. 3 shows examples of types of cells and base stations encompassed by the terminology “low power cell” and “low power base station” as including pico cells and pico base stations, femto cells (which can exist in a femto cluster) and femto base stations, and relay base stations.
The base station nodes (eNBs) are configured with a parameter known as their “cell type”, and this configured information indicates the size of the cell, e.g. “very small”, “small”, “medium”, “large”. However this information is currently not known in the core network nodes such as the mobility management entity (MME).
A new principle has been introduced for location registration in the SAE/LTE networks. This principle is based on a Tracking Area (TA) concept in a similar way as Location Areas (LA) and Routing Areas (RA) in Global System for Mobile communication (GSM) and WCDMA networks. Each SAE/LTE cell belongs to a single TA and an identification of the Tracking Area, known as the Tracking Area Identity (TAI), is broadcasted as part of the System Information. The Tracking Area Identity (TAI) consists of a Mobile Country Code (MCC), a Mobile Network Code (MNC), and a Tracking Area Code (TAC).
The main difference between the Tracking Area (TA) concept and the LA/RA concepts is that in SAE/LTE a further concept called “multiple TAs” or “TAI List” has been introduced. The concept is somewhat similar to the registered zones in cdma2000. This further TAI List concept means that the network may return a TAI List to a user equipment unit (UE) as part of some EMM procedures like Attach, Tracking Area Update (TAU) and GUTI Reallocation. As long as the UE camps on a cell belonging to a TA whose TAI is included in the UE's current TAI List, the UE does not perform normal tracking area updates (TAUs), although periodic TAUs are still performed. The UE performs normal TAU only when it moves to a cell that does not belong to a Tracking Area (TA) in the TAI List. As part of this TAU the UE will receive a new TAI List and the same procedure continues.
As the network knows the UE location for UEs in idle mode (i.e. in RRC-IDLE state) on the TAI List level, this means that the Paging Area is also normally all the TAs included in the TAI List. The Paging procedure is used to inform an UE in RRC-IDLE about an “incoming call” and the need for the UE to move to the RRC-CONNECTED state.
So in an LTE/SAE or EPS (i.e. E-UTRAN and Evolved Packet Core [EPC]) network the user location in idle mode is known in the mobile core network on TAI List level. A Tracking Area (TA) could be the coverage area of one or more cells served by base stations, i.e. eNBs in an E-UTRAN. The MME knows which eNB(s) provide coverage in a Tracking Area (TA), so when a user should be activated, the mobile core network orders the relevant eNB(s) to page the user.
The coverage from a low power cell would provide a relatively small cell within a macro cell as illustrated in FIG. 4. When a UE toggles between a low power cell and a macro cell and these cells belong to different tracking areas (and not to the UE's current TAI list) in the manner shown by way of example in FIG. 4, the core network needs to be informed as described previously by the UE performing a tracking area update (TAU) procedure. This means increased signaling and processing both in the E-UTRAN and in the evolved packet core (EPC). This increased UE signaling also affects the UE battery in a negative way.